This paper presents the results of an extensive theoretical and numerical analysis of different diodes behaviors for energy harvesting (EH) applications, with respect to multitone excitation, in low-power conditions. First, a suitable implementation of the Harmonic Balance technique is adopted to provide a fast and reliable estimation of rectifiers performance under different multi-sine excitations, for increasing number of tones and for different frequency shifts. The chosen operating frequency is 5.8 GHz. A study on the effects of different rectifiers on the RF-to-dc conversion efficiency in the band under multitone excitation, is proposed. The simulations have shown how the circuit parameters of the nonlinear device affect the performance of the rectifier as the number of tones varies. In particular, the focus has been put on a comparative study between circuit simulations and numerical analysis of the performance of rectifiers and their effect on the RF-to-dc conversion efficiency, as a function of their junction capacitance.